Heavy duty construction and earth moving implements such as excavators, cranes, backhoes, loaders and like material handling devices, rely in large measure on hydraulic systems as the motive force to effect their various work functions. In most cases, the output or actuating device of the system is a hydraulic fluid actuator or cylinder (sometimes referred to as a fluid ram) linked directly to the particular working apparatus, such as a loader bucket, a backhoe boom, or the like. The hydraulic pressures utilized in systems of the type discussed typically are quite high, being of the order of 2,000 to 3,000 psi, and under normal operating conditions, the rated hydraulic pressures of a given system are adequate and sufficient to perform the intended operations. Control over the applied pressure is obtained with selectively operable directional control valves which direct pressurized fluid flow from one or more associated hydraulic fluid pumps.
Workers skilled in the art will recognize that the implements frequently are operated in modes or positions which can tax even the maximum rated pressure of the system, such as when "craning" with an upraised heavy load. For example, during the typical operation of a backhoe or loader, the bucket of the implement may be filled with or attached to a load, and then moved to an extended or upraised position. Frequently, the implement in that condition is then required to travel some distance before releasing the load. If, as is often the case, the ground over which the implement must travel is bumpy or uneven, the shocks and jolts transmitted to the extended and loaded bucket can well create fluid pressure forces approaching the maximum rated pressure of the hydraulic system. Under such conditions, failure of a hydraulic fluid line or other component could result in undesired loss of control of the supported load. Even if the load forces are not excessive, the normal hydraulic fluid control valves may be unable to provide the desired degree of accuracy of hydraulic fluid flow control during such operations.
There thus exists a need for a means to accommodate operation of an implement's hydraulic system at high load levels while maintaining desired control of a supported load. More specifically, there is a need for means capable of selectively locking the system into a load-supporting condition so that temporary overload conditions will have no appreciable effect on the implement or the work function then being performed.